1. Field of the Invention
This invention generally relates to compositions comprising cationically polymerizable materials which are curable in a short time, e.g., fixturing in approximately one minute or less and reaching full properties within a short time thereafter, in the manner of cyanoacrylates.
More specifically, the present invention relates to curable compositions comprising cationically polymerizable materials wherein rapid cure is achieved by the in situ generation of cations, which function as catalytic species for the cure of such materials, and to two-component adhesive compositions which in use are applied one component to each of two matable surfaces with rapid cure occurring on mating of the surfaces.
2. Description of the Related Art
As discussed in our copending U.S. patent application Ser. No. 669,189, filed Nov. 7, 1984, a major disadvantage of prior art fast cure epoxies curable at room temperature conditions, e.g.. mercaptan cured epoxies, is that rapid gellation of such materials retards the development of full properties due to the restriction of molecular motion and the cure mechanism involved. Extremely fast cures in these compositions result in overly plasticized products due to the need to use a high level of the curative component. Further, for conventional amine- or mercaptan-cured epoxies, there is a critical mix ratio or stoichiometry for the epoxy resin/curing agent (hardener) mixture, with thorough mixing necessary to achieve full cure.
In the aforementioned copending patent application, we have described storage stable dispersions of stable carbocation salts as initiators for polymerizing epoxy resins to high molecular weight solids. Specifically disclosed in such application are two-part adhesive compositions of a type exhibiting rapid room temperature cure, which in a first part comprise an oxirane functional resin and in a second part comprise a stable carbenium ion salt, i.e., a triphenylmethyl or tropylium salt in a suitable carrier.
In epoxy compositions of the type exemplified in the above-described application, carbocation initiators cause spontaneous epoxy polymerization and thus generally are dispersed in an unreactive carrrier; as such, the respective components require thorough mixing to produce a homogeneous polymeric solid. In many instances, the requirement of such thorough mixing is at odds with the rapid cure nature of the composition, and may result in premature fixturing of the composition prior to its application to the desired substrate.
Illustrative of the prior art usage of other curing agents which effect spontaneous polymerization when mixed with a separate epoxy resin component are U.S. Pat. No. 4,396,754, which teaches an epoxy curing catalyst selected from lithium or Group II metal salts of a non-nucleophilic acid, including LiBF.sub.4, Ca(BF.sub.4).sub.2, Mg(BF.sub.4).sub.2, LiPF.sub.6, Ca(PF.sub.6).sub.2, Mg(PF.sub.6).sub.2, LiSbF, LiAsF.sub.6 and the like, and U.S. Pat. No. 4,092,296, which teaches to utilize metallic fluoroborate salts, e.g., salts of tin, copper, zinc, nickel, lead and silver, in aqueous solution as epoxy curing agents. See also U.S. Pat. Nos. 4,069,055 and 4,138,255 to J. V. Crivello which disclose the cationic polymerization of epoxy resins by radiation sensitive aromatic onium salts of Group VA and VIA.
The aforementioned U.S. Pat. No. 4,092,296 teaches the use of silver tetrafluoroborate but shows it by example to be a poor single catalyst (Examples 192-197; see column 23, line 51-column 24, line 22). All other metallic salts disclosed in the U.S. Pat. Nos. 4,396,754 and 4,092,296 patents are characterized by short shelf lives due to their activity as single catalysts, i.e., their spontaneous high reactivity with epoxy resins.
In our copending U.S. patent application Ser. No. 821,570 filed Jan. 22, 1986, there is disclosed a two-part composition curable by contact of respective parts with one another, comprising: a first part comprising a cationically polymerizable material and a first latent curing component; and a second part comprising a cationically polymerizable material and a second latent curing component which is reactive with the first latent curing component to form cations which are curingly effective for the cationically polymerizable material.
The curingly effective cations may comprise carbenium ions, zwitterions, protons, hydronium ions, or any other positive ions which are useful to cationically polymerize the polymerizable material in the two-part composition, such as an epoxy resin. Such curingly effective cations may be formed by addition, dissociation, or charge transfer initiation reactions between the respective first and second latent curing components.
Various first and second latent curing component species are described in the aforementioned application, including preferred compositions where the first latent curing component is selected from the group consisting of organic halides and water, and the second latent curing component is a silver salt of a non-nucleophilic anion selected from the group consisting of: SbCl.sub.6.sup.-, SbCl.sub.3.sup.-, SbF.sub.6.sup.-, AsF.sub.6.sup.-, SnCl.sub.5.sup.-, PF.sub.6.sup.-, BF.sub.4.sup.-, CF.sub.3 SO.sub.3.sup.-, and ClO.sub.4.sup.-.
In use, the above-described two-part compositions are applied one part to each of two matable surfaces, which upon mating of the surfaces results in rapid cure occuring to adhesively bond the respective substrate surfaces. Specifically, when the coated surfaces are mated, the organic halide or water component is dissociated by the silver salt, thereby complexing or otherwise precipitating the halide moiety of the organic halide when same is used as a first latent curing component and forming a carbocationic curing species, or in the case of water, dissociating same to form hydronium ions and/or protonic cationic species effective for curing the cationically polymerizable material. Two-part adhesive compositions of such type utilizing oxirane functional resins as the cationically polymerizable material, e.g., a cycloaliphatic resin, exhibit extremely fast fixturing times, on the order of one minute or less.
One problem with the foregoing compositions is that the silver salts are readily reactive with moisture, resulting in the generation of hydronium ions and/or protons (H.sub.3 O.sup.+ /H.sup.+) which are curingly effective for the resin component of the composition, so that water is usefully employed as a latent curing component in combination therewith. Nonetheless, any atmospheric moisture or water contaminant coming into contact with the silver salt in the silver salt-containing part of the two-part composition, prior to use thereof, may result in undesired premature curing and gellation of the associated resin in such part of the two-part composition.
Accordingly, Ser. No. 821,570 teaches (e.g., at page 17, lines 5-9) to dry the cationically polymerizable material with zeolitic driers, to provide good shelf life "in excess of two months."
Shelf life and moisture susceptibility of the silver salt-containing part are critical factors in the commercial utility of the aforementioned two-part compositions. It is known that the shelf life of the silver-salt containing part of the composition is measurably decreased by exposure to moisture. It would therefore be a significant advance in the art, in view of the superior performance and properties of the two-part compositions disclosed in the aforementioned application, to increase the moisture resistance of the metal salt-containing part to extend its shelf life.
It is therefore an object of the present invention to provide a co-curable composition, having utility as a first part of a two-part composition for contacting with a second part comprising a cationically polymerizable material and a latent curing component, wherein the co-curable composition comprises a cationically polymerizable material and a metal salt of a non-nucleophilic anion, and is characterized by high moisture resistance.
It is another object of the invention to provide a two-part composition comprising as a first part the above-described co-curable composition, and a second part comprising a cationically polymerizable material and a latent curing component which is reactive with the metal salt to form curingly effective cations for the cationically polymerizable material.
Other objects and advantages of the present invention will be apparent from the ensuing disclosure and appended claims.